Some inner diameter caliper pigs are allowed to run within a pipeline to measure the inner diameter of the pipeline or to detect protruded and recessed portions on the inner circumferential surface of the pipeline and deformations of the pipeline and the like. A seal cup of such an inner diameter caliper pig is formed of a resilient material and has a property of being dented about 10% of the diameter. There is known a mechanical apparatus which measures the deformation of such a seal cup to detect a deformation of the pipeline interior. For example, there is available a mechanical apparatus with a skeletal structure which is formed in the shape of a “Karakasa,” a Japanese paper umbrella, capable of being opened or closed, within the inner surface of the seal cup.
Disclosed in U.S. Pat. No. 4,953,412 is a technique for measuring a deformation of a seal cup with a mechanical measuring device. FIG. 11 illustrates such a mechanical measuring device of a prior art inner diameter caliper pig. This device is provided with levers 103 for detecting a displacement near the edge of a seal cup 101 of a pig 100 running within a pipeline and adapted to convey their movements by means of link mechanisms 106. For example, there are twelve levers 103 arranged along the circumference. The lever 103 is pivotally attached at one end to a pig body frame 102 with a pin 104 and provided at the other end with a contact probe 105 which is in contact with the reverse surface of the seal cup 101 near the edge thereof to sense the movement of the edge of the seal cup. The link 106 is coupled to the central portion of the lever to sense the movement of the lever 103. The other end of the link 106 is coupled to a disc 107 near the outer circumference thereof. The disc 107 is movable with respect to a bracket 108 that is generally perpendicular to the pig axis and supports the disc at its center. The disc 107 is tilted about the bracket 108 in response to the movement of the link 106. The tilting movement of the disc 107 about the bracket 108 is detected by the axial movement of a plurality of points on the disc 107. As this mechanism, the figure shows only a coupling shaft 109 and a detection mechanism 110 for detecting the movement of the central portion, not illustrating the detection mechanisms at other points on the disc which, however, are similar to the one illustrated. The detection mechanism 110 for detecting the axial movement includes an electrical transducer, so that the extent of displacement is transformed into electrical signals for storage.
The device transforms the movement of the contact probe 105 (for sensing a variation in the interior wall of a pipeline, i.e., a deformation of the seal cup 101 near the edge thereof) into a tilting movement of the disc 107 for detection. Therefore, only information on an average tendency can be obtained. For example, it is not possible to determine whether a variation in the inner diameter of the pipeline is due to a weld bead or a partial dent. It is also impossible to determine the orientation of a dent in the pipeline. Furthermore, an elliptical deformation in the inner diameter of the pipeline could not be sensed in a specific manner.
The present invention is intended to provide a technique which can improve the prior art inner diameter caliper pig to measure with high accuracy a deformation of a seal cup running within a pipeline; which can distinguish a weld bead from other deformations so as to provide information as to the circumferential location of the deformation; and which can detect the inner diameter of a pipeline even when deformed into an elliptical shape or other shapes.